How To

Dark Slippery Areas on Slab

Q.: I have two projects that have developed darkened and slippery sections on the floor slab surface. One project is 12 months old and the second is 2 years. When certain environmental conditions appear, moisture condenses on the surface and tends to turn into a slippery material (more than just water). For this slab there were two distinct floor pours. One section had a bond breaker then a sodium silicate hardener applied. The problems seem to occur primarily where we have the two products. We have repeatedly cleaned the floors to attempt removal of any bond beaker residue but problems still prevail. We are considering etching the slab, either chemically or mechanically to allow the concrete to absorb any moisture that condenses on the floor. Do you have experience with similar problems?

A.: We know of a warehouse slab that has so much condensation on the surface that the forklift would spin its wheels on the regular hard trowel finish. This phenomenon primarily occurs in the summertime when the relative humidity is high. Any or all of the following factors, according to the expert that was hired, could cause the floor to condense atmospheric moisture into dew: no vapor barrier, no stone or gravel subgrade (in other words, the slab was poured against a clay or silt subgrade), or no air conditioning in the summer. The recommended solution was to have an air conditioning system installed with sufficient capacity to dry the warehouse space.

The surface problem, however, may be with the sodium silicate hardener becoming slippery when wet. The best way to remove excess sealer material is with wet scrubbing. Wash water is required to keep the excess sodium silicate wet enough to remove. A chemically treated floor develops a waxy sheen with time. The sodium silicate hardeners significantly reduce surface porosity, so you would have to grind the surface to the full depth of penetration (2 to 5 mm) to get more absorptive concrete. You should check this with core samples before attempting any surface prep.

We would also question whether or not the concrete contained calcium chloride as an accelerator. Unfortunately the calcium will also absorb water from the air and under high humidity conditions darken the surface. In high enough concentrations it has been known to cause "condensation" on the surface. Here’s a suggestion: tape a 1-foot square of polyethylene film to the floor and leave it for a couple of days. If there is water under the poly, then it is moisture passing through the floor. If there is none, but moisture is present elsewhere, then it is being absorbed from the air, and calcium chloride is one possibility. Excess calcium chloride can sometimes be removed by layering the floor with new paper, soaking it with water, and then allowing it to dry. This can help to pull the calcium out of the surface concrete. Throw the paper out and repeat if necessary (it may take a number of tries.)